In reality, there’s always some voltage at the output, called the offset voltage, VOS. If the offset voltage at the output is divided by the noise gain of the circuit, the result is called the input offset voltage or input-referred offset voltage. VOS drifts with temperature, and TCVOS is the temperature coefficient of that drift.
Although ideal amplifiers have infinite input impedance, and, theoretically, no current flows into their input terminals, real amps that use bipolar junction transistors (BJTs) in the input stage require bias current (IB) for operation.
Total harmonic distortion (THD) reflects to the harmonically related components of the fundamental frequency caused by amplifier nonlinearity. Usually, only the second and third harmonics need to be considered. THD+N (THD plus noise) accounts for device noise.
THD and THD+N are both measurements of distortion generated by a single-tone sine-wave input. Intermodulation distortion (IMD) is a measurement of dynamic range produced by the interaction of two tones. Third-order intercept point (IP3) is a measure of the effects of third-order IMD.
The 1-dB compression point represents the level of input signal at which the output signal is compressed by 1 dB from an ideal input/output transfer function. This defines the end of an amp’s dynamic range. Signal-to-noise ratio (SNR) also defines the dynamic range. It is a measurement (in dB) from the maximum signal level to the RMS level of the noise floor.
In RF work, noise factor and noise figure are important specs. Noise factor relates the noise generated by the amplifier to the thermal noise of a 50-O resistor at room temperature. A noise factor of 2 means the amplifier is as noisy as a 50-O resistor. Noise figure is noise factor expressed in dB, i.e., 10 × log10 (noise factor).
DEVICES AND MATERIALS For instrumentation applications, audio work, and RF up to VHF, conventional bipolars and field-effect transistors fabricated using conventional process technologies are the rule. The only difference between these processes and the ones used for mainstream digital ICs is that the analog parts tend to lag several generations behind in terms of design rules. That’s because it’s difficult to deal with system noise with amplifier input voltage swings below 3.5 V. On the other hand, at higher RF frequencies, exotic materials and advanced transistor architectures dominate. iSilicon-germanium (SiGe) is a silicon bipolar process technology in which the transistor bases are doped with germanium. To illustrate the advantages of SiGe in RF amplifiers, consider Maxim Integrated Products’ GST-3 process, a SiGe extension of its silicon GST-2 process. GST-3 offers an important decrease in transistor parasitic base resistance (RBB´) and a significant increase in ß.
In terms of noise figure, adding germanium to the p-silicon base of a transistor reduces the bandgap by 80 to 100 mV across the base, creating a strong electric field between the emitter and collector junctions. By rapidly sweeping electrons from the base into the collector, this electric field reduces the transit time (tB) required for carriers to cross the narrow base.
If all other factors are held constant, this reduced tB provides an approximate 30% increase in cutoff frequency (fT). Higher fT reduces high-frequency noise because the ß rolloff occurs at a higher frequency. For identical-area transistors, the SiGe device achieves a given fT with one-half to one-third the current required in the puresilicon device.
Maxim also notes that SiGe bipolars also require lower supply currents and provide higher linearity than conventional silicon transistors. Given high production volumes, SiGe devices are inexpensive and nearly as good as gallium arsenide (GaAs) in terms of noise figure and power. However, the higher the operating frequency, the lower the breakdown voltage, and hence the operating voltage.
The first microwave amplifiers were heterojunction bipolar transistor (HBT) metal epitaxial semiconductor field-effect transistors (MESFETs) and high electron mobility transistor (HEMT) MOSFETs, both built using GaAs process technologies. MESFETs are like junction FETs, but with a Schottky (metal-semiconductor) junction. HBTs use different semiconductor materials for the base and emitter. HEMTs are more common.
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